Journal of the Marine Biological Association of India

Volume 15 Issue 1

Continental drift and the distribution of the island floras of the Indian Ocean.

R. Melville

The floras of the low lying archipelagos of the Indian Ocean are of a uniform type and can be accounted for by trans-ocean colonisation, but those of the high islands are diversified, with high degrees of endemism, signifying long periods of isolation. An explanation of this situation can be found in continental drift. As the land masses surrounding the Indian Ocean were formerly parts of the Gondwana continent of late paleozoic and mesozoic times, an understanding of the former arrangement of the fragments and the manner in which they broke apart from one another is necessary to account for modern biological distributions. Many attempts at reconstruction of the Gondwana continent have been made by geologists and geophysicists, but generally without taking the palaeontological evidence into account. A new reconstruction of Gondwanaland is proposed and the sequene of its disruption in the Indian region discussed. This suggests that Madagascar was separated from both Africa and India in the first phase of the drift process but remained close to India until India broke away from the remaining parts of Gondwanaland. The Mascarenes were probably formed along the mid oceanic ridge at an early stage in the drift of India when it was close to Madagascar. The distributions resulting from these earth movements include those of the Canellaceae linking Madagascar with Africa and the West Indies on the one side and Adansonia on the other linking Africa through Madagascar to Australia. The close relationship between the Mascarene floras may be due to temporary land connection along the mid ocean rise at an early stage in the drift, later broken by subsidence. Some examples are discussed.

The high rate of endemism in the flora of Madagascar is a reflection of its long isolation , which has also allowed the survival of some primitive features. Such are the glossopteroid leaves of Foetidia, replaced by a more advanced type in the Mauritian species, and they give an indication of the interest and scientific importance of the islands of the Western Indian Ocean for phytogeographical and evolutionary studies.

Date : 31-08-1973